This invention relates to a method of making a composite product from a finely divided lignoccllulosic material and a suitable hydraulic binder, and to the composite product so made.
Cement bound wood wool and cement bound particle board are well known. Cellular or aerated cents are equally well known, the cells being formed either by foaming agents in water, the foam being added to a cement paste, or alternatively the cells being formed in the cement by an agent evolving gas in an alkaline medium.
An example of a foamed building board is disclosed in South African Patent No 92/6179 (corresponding to U.S. Pat. No. 5,395,571). This patent teaches a method of making a foamed building board or the like from the following components:
(a) a major amount of weight of an inorganic base material selected from the group consisting of a calcium sulphate hemihydrate, magnesium oxychoride, magnesium oxysulphace and a hydraulic cement;
(b) a suitable amount by weight of the inorganic base material of a thermosetting resin which is miscible, soluble or dispersable in water;
(c) a suitable amount of a catalyst for the thermosetting resin:
(d) water in an amount sufficient to rehydrate the inorganic base material with the water present in the other components;
(e) optionally a suitable amount of a plasticizer such as a melamine formaldehyde condensate;
(f) optionally a suitable amount of a polyvinyl alcohol;
(g) optionally a suitable amount of a retarder for the setting time of the inorganic base material;
(h) optionally a suitable amount of a fibrous reinforcing material; and
(i) a suitable amount by weight of a foam or a foaming agent;
which method comprises the steps of:
(1) mixing together components (a), (b), (c) and (d), and (e), (f) and (g) if present;
(2) adding component (h) if present into the mixture of step (1);
(3) adding component (i) into the mixture of step (2) with stirring to give a foamed product;
(4) forming the product of step (3) into a building board; and
(5) allowing rehydration of the inorganic base material to occur and curing of the thermosetting resin to occur.
Cement bound lignocellulosic composites can suffer from the disadvantage that the celluloses and the hemi celluloses in the lignocellulosic material are attacked by the alkaline components of the cement, and soluble sugars can retard Portland cement hydration. In addition, in foamed or cellular cement composites, cracking due to shrinkage and excessive brittleness can occur. A further disadvantage of cellular cement bound lignocellulosic composites is the cost of the foaming agent. Further, the extended setting time of, for example, a Portland cement can result in partial or total subsidence of the heavier fractions in the foamed mix resulting in variable cell size, or even major voids. Difficulty in process parameter controls can result in a product of variable density, consistency and performance.
There is a need for a new composite product containing a lignocellulosic material and a hydraulic binder.
According to a first aspect of the invention there is provided a method of making a composite product including the steps of:
(a) mixing:
(i) a hydraulic binder,
(ii) finely divided lignocellulosic fibres in an amount of from 1% to 30% inclusive by mass of the hydraulic binder; and
(iii) water optionally containing a polyvinyl alcohol, the water being present in an amount sufficient to form a paste;
(b) introducing a foam generated from a polyvinyl alcohol into the paste, in an amount of from 0.5% to 15% inclusive by mass of the paste and mixing to form a foamed product;
(c) forming the foamed product into a desired shape; and
(d) allowing the hydraulic binder to set to form the composite product.
It is to be noted that the composite product is formed in the absence of a thermosetting resin. In other words the composite product contains no thermosetting resin.
Lignocellulosic material refers to any plant material emanating from be photosynthetic phenomenon. This includes paper, linen, cotton, hessian, and the like.
By finely divided lignocellulosic fibres, there is meant unifibres, i.e since fibres, or bundles of a small number of unifibres of the lignocellulosic material. In other words, the lignocellulosic material is broken down into single or unifibres or bundles of a small number of fibres, rather than being in chip or particle form. This is necessary as the function of the finely divided lignocellulosic fibres is to act as a theological controller and foam stabiliser, and as a reinforcing fibre strengthened by the polyvinyl alcohol.
The finely divided lignocellulosic fibres are preferably obtained from paper mill sludge, paper waste or refined fibres such as are used in medium density fibreboard manufacture.
The hydraulic binder may be selected from the group consisting of a hydraulic cement, such as a Portland cement e.g ordinary Portland cement or Rapid Hardening Portland cement, a calcium sulphoaluminate cement, a high alumina cement, a gypsum cement, cacium sulphate hemihydrate in either the alpha or beta form, an alkali silicate, magnesiumn oxychloride, and magnesium oxysulphate, and mixtures of two or more thereof. The preferred hydraulic binder is gypsum, i.e calcium sulphate hemihydrate in either the alpha or beta form.
The finely divided lignocellulosic fibres are preferably used in an amount of from about 2.5% to about 15%, more preferably from about 3% to about 12% inclusive by mass of the hydraulic binder.
The finely divided lignocellulosic fibres and the hydraulic binder are mixed either with water or with a solution of water and a polyvinyl alcohol. In the latter case, the solution may contain about 1% to about 10% inclusive of the polyvinyl alcohol on the mass of the water.
The finely divided lignocellulosic fibres and the hydraulic binder must be mixed with sufficient of the water or the water/polyvinyl solution to form a paste.
In step (a) the finely divided lignocellulosic fibres may be dispersed in water or the solution of water and a polyvinyl alcohol, and thereafter the hydraulic binder in dry form may be added thereto and mixed to form the paste.
Alternatively in step (a) the finely divided lignocellulosic fibres in dry form may be mixed with the hydraulic binder in dry form, and thereafter water or the solution of water and a polyvinyl alcohol may be added thereto and mixed to form the paste.
The method of the invention may include a step, between step (a) and (b) of:
(e) immersing particles of a lignocellulosic material in water or in a solution of a polyvinyl alcohol in water, removing the surplus water or solution, and blending the particles with the paste of step (a).
By particles of a lignocellulosic material there is meant chips, strands or flakes of a lignocellulosic material, i.e particles which are much larger in size than the size of the finely divided lignocellulosic fibres, and which are intended as a filler material and not as a theological controller and foam stabiliser.
The solution of the polyvinyl alcohol in water preferably comprises an amount of from about 1% to about 10% of the polyvinyl alcohol by mass on the mass of the water.
In step (b), the foam is preferably generated by injecting air into a moving stream of a polyvinyl alcohol solution in water, the solution containing from about 1% to about 18% by mass inclusive of the polyvinyl alcohol, on the total mass of the solution, more preferably an amount of about 2% to about 7% by mass inclusive of the polyvinyl alcohol on the total mass of the solution.
A suitable surfactant may be added to the polyvinyl alcohol solution in order to lower surface tension and to propagate a fine cellular foam. Suitable surfactants include the silicone glycol copolymers such as DC193 or DC197 by Dow Corning
The amount of foam added relative to the total mass of the paste, i.e the total mass of the lignocellulosic fibres and particles (if present), the hydraulic binder and water, is at a level of from 0.05% to 15% inclusive, preferably in an amount of from 1.5% to 8% inclusive by mass, more preferably in an amount of from 1.5% to 5% inclusive by mass.
In step (c), the foamed product may be cast or poured or otherwise dispensed into a suitable mould or casting receptacle or the like.
In step (d), the hydraulic binder is allowed to set, with or without acceleration promoted by heat induction or catalysis, to form the composite product.
Thereafter, the composite product may be used directly, or may be cut into smaller sections for use.
According to a second aspect of the invention there is provided a composite product made by the method set out above.